Immunohistochemical assays and assay techniques based on in situ hybridization are widely used in medical diagnostics such as to diagnose abnormal cells such as those found in cancerous tumors or to diagnose another disease. Many assays involve the addition of heat to the sample, such as to a sample in a reagent on a slide. Assay steps at elevated temperatures can cause substantial evaporation of assay reagents. When small volumes are used, sample denaturation, hybridization, wash and aging steps and the resulting assay can be compromised by evaporation.